The present invention generally relates to power converters having magnetic amplifier (magamp) post regulators and, more particularly, to circuitry used to reduce the power and efficiency loss in the control transistor of a set-mode magamp post regulator.
The magamp post regulator is a popular power supply topology for regulating the outputs of a power converter in many applications. Modern electronic devices often require several voltage outputs; and need a low cost, energy efficient and well regulated way of providing these outputs. Magamps are typically used to provide an efficient and reliable way of providing precise voltage regulation of independent outputs of a multiple output power converter. A magamp post regulator provides improved regulation of power converter output voltage using a small control current.
The basis function of a magamp is to block a positive incoming voltage for a certain time (tblock) before allowing it to pass through an output filter. The duty cycle reduction occurs because the magamp delays the leading edge of the voltage waveform. The magamp acts to reduce the duty cycle to the rest of the circuit from the duty cycle of the incoming voltage so as to maintain the required average output voltage.
Conventional magamp post regulator circuits use a reset control to control the magamp using a control transistor operated in a linear mode. FIG. 1 illustrates a prior art example of a conventional reset-controlled magamp circuit 10. FIG. 2 illustrates the hysteresis characteristic of the core element of the magamp of the circuit of FIG. 1. The conventional magamp circuit includes a magamp 16, a diode 12, a reset transistor 20 and an error amplifier (error amp) 18. In FIG. 1, when a power switch 34 is turned on, a secondary voltage Vsec is developed across a transformer 14 secondary winding. Magamp 16 is forced into saturation due to the action of the voltage Vsec forced upon it. The B-H hysteresis curve in FIG. 2 shows the saturation point, Bsaturation at the top of the path. Since the magamp 16 is xe2x80x9cin saturationxe2x80x9d, forward biased and highly conductive, current flows through the magamp to a forward output rectifier diode 22 after which it is filtered by an L-C circuit, comprised of inductor 24 and capacitor 26. The output voltage is coupled to a load, not shown, and is also divided by a voltage divider formed by series resistors 36 and 38 to generate a Voltage sense signal at node 35. At the end of the switch xe2x80x9conxe2x80x9d time, the magamp 16 remains forward biased and in saturation.
When the main power switch 34 turns off and the transformer 14 voltage reverses polarity to xe2x88x92Vsec the current through the magamp 16 is caused to ramp down. As a result, a vertical rectifier diode 30 must pick up the output current, causing the voltage at node 15 to drop. In this off state the magamp voltage Vm is not allowed to reach zero. Instead a reset control circuitry supplies a voltage that reversely biases the magamp 16, such that the magnetic flux density is reset to a point below remanence (below the point Bremanence of the left side of dark shaded area in FIG. 2.). Then the main switch is turned on and the transformer 14 secondary voltage becomes +Vsec. Since Magamp 16 is well below the saturation point and not conductive, it acts as an open circuit and blocks the secondary voltage. Vertical diode 30 continues to provide a path for the output current so the voltage at node 15 remains at zero. The magamp voltage Vm then equals +Vsec. In time, the voltage across magamp 16 causes it to reach saturation and become conductive. The current through the magamp rises to the output current level and remaining at this level till the end of the on time.
The flux excursion on the B-H curve of FIG. 2 depends on how much volt-time is applied across the magamp 16 during resetting. The amount of volt-seconds is controlled by the output of error amp 18. The blocking time equation is given by             t      block        =                  Δ        ⁢                  xe2x80x83                ⁢                  B          ·          turns          ·                      A            core                              V        ;
where Acore is the core area, xcex94B is the change in flux density, turns is the number of turns for the core, and V is the voltage. It can be seen from this equation that the loop in FIG. 2 corresponding to xcex94B2 gives a longer blocking time that the loop of xcex94B1. The cores required for this prior art method of reset control exhibit a relatively square B-H curve. To lower the output voltage and increase the blocking time, the loop followed is the lightly shaded part of the B-H curve as compared to the dark part. The control circuit forces the B-H loop larger by pushing the vertical, descending part of the locus. Thus, the minimum blocking voltage-time is the locus where it just touches the vertical axis. To maximize the difference between maximum and minimum volt-time blocking, the B-H loop of the core material must have a small difference between Bsaturation and Bremanence, where it intercepts the vertical axis.
Compared to square loop amorphous core magamps, ferrite magamps are lower cost, better for high frequencies and can run at higher temperature. However, a drawback associated with this conventional reset control approach is that lower cost non-square ferrite cores perform poorly under reset control because the power dissipation at high flux excursion is too large, especially for operation at high frequency.
A prior art example of a conventional circuit for magamp post regulator control without using reset control but instead using a xe2x80x9csetxe2x80x9d mode with a control circuit in a linear mode, is shown in FIG. 3. This set control enables the use of lower cost ferrite cores for the magamp core, however, operation in linear mode leads to unacceptable losses in the circuit. The corresponding B-H hysteresis characteristic of the core member of the magamp of the circuit is shown in FIG. 4. For the magamp post regulator 40 in FIG. 3, an error amp 48 feeds a control transistor 50 which is operated in linear mode. When the transformer 44 secondary voltage Vsec turns negative in response to power switch 64, a diode 42 and a control transistor 50 xe2x80x9ccatchxe2x80x9d the current through magamp 46. Depending on the voltage output from error amp 48, the current through the loop of diode 42, control transistor 50 and magamp 46 is decreased, and the corresponding change in xcex94H and xcex94B is achieved (as shown in FIG. 4, the current is related to H by the equation H*Lcore=turns * I.) During the next positive cycle, the magamp 46 will block the secondary voltage Vsec The blocking time, Tblock, according to the equation described above,             t      block        =                  Δ        ⁢                  xe2x80x83                ⁢                  B          ·          turns          ·                      A            core                              V        ,
is proportional to xcex94B (turns, Acore and V are constant for the equation). As the curve in FIG. 4 illustrates, set control mode operates only at one quadrant of the B-H curve while the reset control, as shown in FIG. 2, can operate at all four quadrants. In this xe2x80x9csetxe2x80x9d mode circuit, the control circuit tries to prevent the core from resetting, i.e. tries to make a smaller loop. Since there is no requirement for the core to be square, non-square less costly ferrites can be used.
FIG. 5 shows another prior art version of set control for a magamp post regulator. For this magamp post regulator circuit 70, in addition to the magamp 76 power winding, there is an extra magamp control winding 77. A driver diode 72 and a control transistor 80 control the magamp control winding 77, with the control elements isolated from the transformer 74 secondary power winding. The current through the diode 72 and control transistor 80 can be reduced depending on the turns ratio of the control winding and power winding. FIG. 6 shows a corresponding set of timing curves for the magamp set control circuit of FIG. 5. The top curve 1, is the secondary voltage and Vp is the transformer 74 primary voltage, curve 2 is the Verror voltage, curve 3 is the transistor 80 collector-emitter voltage, Vcc, and curve 4 is the magamp voltage Vm. FIG. 7 shows a set of measured voltage curve traces for the magamp set control circuit of FIG. 5. Curve 5 is the secondary voltage, curve 6 is the voltage at the anode of the horizontal diode 82 and the lower curve 7 is the magamp voltage Vm.
The conventional set control circuits of FIGS. 3 and 5 allow the use of lower cost non-square ferrites. A drawback of these circuits, however, is that the circuits exhibit unacceptable power and efficiency loss. FIG. 8 illustrates the unacceptable energy loss. The stored energy in the core is the area bounded by the B-H curve and the B axis. When traversing the lower part of the B-H curve up to saturation, the energy stored is equal to the light shaded area Ae plus the dark shaded area Ah;with Ah representing the energy lost due to hysteresis. When traversing the curve from saturation to the area between saturation and remanence, a part of the area Ae is associated with the movement. Under set control, the energy is dissipated in the control transistor.
FIG. 9 is a set of measured trace curves illustrating the power dissipation drawback of the conventional set mode circuits. Curve 8 is the secondary voltage Vsec, curve 9 is the current for the control transistor and 10 is the control transistor voltage. The voltage and current waveforms between the vertical cursors illustrate that the power is being dissipated in the control transistor that is operating in its linear region. At higher power levels, more power will be dissipated. Under set mode control, the energy has been found to be dissipated in the control transistor that is the driver element for the magamp post regulator.
To allow the use of any kind of loop material regardless of its residual flux and to use ferrites effectively at lower frequencies, a conventional xe2x80x9cfull controlxe2x80x9d method has also been used. For this full control method, both the reset and set control methods are selectively used; with either being applied to the same core.
A drawback associated with the xe2x80x9csetxe2x80x9d control and the xe2x80x9cfull controlxe2x80x9d methods, as described above, is that losses in the control transistor are quite high, resulting in unacceptable reductions in power and efficiency. Parasitic energy stored in the magamp during the power delivery is burned in the control transistor. Therefore, there is a need for circuitry to reduce this power and efficiency loss in the control transistor of a set mode magamp post regulator circuit.
The aforementioned drawbacks associated with losses in the control transistor in xe2x80x9csetxe2x80x9d mode magamp post regulators are substantially reduced or eliminated by the present invention. One aspect of the present invention is directed to a switched set mode magamp post regulator circuit operative to eliminate the power loss associated with operation of the control transistor in linear mode, by switching the control transistor on and off synchronously with the main transformer. The switched magamp post regulator circuit enables the parasitic energy stored in the magamp to be recycled to the output load. The switched magamp circuit also reduces cost over the more commonly used reset-mode magamp circuits by employing the set mode which enables the use of different materials for the magamp core. The magamp post regulator control circuit embodiments described below are for regulating one or more output voltages of a power converter. The embodiments are described where the power converter is a forward converter, however, the present invention is equally applicable to other topologies including push-pull, half-bridge, full bridge and flyback; especially when there is a periodic rectangular voltage source similar to the Vsec transformer secondary waveform.
One exemplary embodiment of the present invention, shown in FIG. 10, provides a set mode magamp post regulator control circuit for regulating the output voltage of a power converter. The magamp post regulator circuit comprises a magnetic amplifier, a control transistor, a set mode control circuit and an output circuit. In this embodiment the control transistor is preferably a MOSFET. A power switch signal is turned xe2x80x9conxe2x80x9d a secondary voltage Vsec is developed across the transformer winding. A set mode control circuit switches the control transistor on and off synchronously with the main transformer. A winding on the magamp is allowed to xe2x80x9cflyxe2x80x9d then subsequently gets shorted out, during every cycle, during the off-time for the primary of the power converter, in order to get the desired B-H excursion curve of the magamp core. When the control transistor is off, the energy from the magamp is returned to the load. When the control transistor turns on later in the cycle, current will circulate in the control windings of the magamp. The magamp preferably includes multiple magamp windings and a low-cost ferrite core.
An advantage of this embodiment is that the use of a switching mode of operation improves the power and efficiency by reducing losses in the control transistor compared to conventional circuits in which the control transistor is operated in linear mode. The set mode also allows the efficient use of lower cost core materials including ferrites.
In another exemplary embodiment of the present invention shown in FIG. 13, the switched magamp post regulator control circuit uses set mode control with a feedback control using pulse width modulation (PWM). The magamp post regulator control circuit comprises a magnetic amplifier, a control transistor, a control circuit and an output circuit. The control transistor to be switched on and off is also preferably a MOSFET. The control circuit for switching the control transistor is comprised of a comparator, an error amp and a ramp generator circuit. When the control transistor is off, the energy from the magamp is returned to the load. When the control transistor turns on later in the cycle, current will circulate in the control windings of the magamp.
FIG. 15 shows the preferred embodiment of the magamp post regulator circuit of FIG. 13. There are two main differences between FIGS. 13 and 15. One is that the ramp voltage waveform, Vramp, produced in the embodiment in FIG. 13 is triangular whereas the ramp voltage waveform produced in FIG. 15 is trapezoidal. Secondly, for the embodiment in FIG. 15, the voltage at the negative input of the comparator passes through a diode and is DC biased. The DC bias feature incorporated into FIG. 15 is essential to ensure that the MOSFET control transistor is off during the time when the secondary voltage Vs is positive even in cases wherein the output error voltage goes to its lowest possible voltage. Note that in FIG. 13 if the error amp 208 is saturated and the ramp transistor 211 is fully on, the output of the comparator 221 is unpredictable and would be dependent on which of the two voltages is larger. The trapezoidal waveform in the embodiment in FIG. 15 raises the effective error voltage needed to operate in the ramp""s dynamic range. This makes the circuit more immune to false triggering. Note also that FIG. 15 easily allows the addition of another error amp circuit for constant current operation if needed. The embodiment may optionally include a drive circuit to drive the control transistor. The embodiment in FIG. 15 shows the error amp portion of the control circuit configured xe2x80x9cfor constant voltagexe2x80x9d control.
FIG. 18 shows an alternative embodiment of FIG. 15 with an error amp circuit that provides both constant voltage and constant current control. Alternatively the constant current control, shown in FIG. 17, could be provided without the constant voltage control circuit.
An alternate embodiment of the present invention shown in FIG. 19, provides xe2x80x9cfull controlxe2x80x9d over the magamp post regulator control circuit for regulating the output voltage of a power converter. xe2x80x9cFull controlxe2x80x9d refers to control over the full range of the hysteresis loop [from xe2x88x92Bsaturation to +Bsaturation]. Unlike the conventional full control circuits, this embodiment uses both a set mode (switched) and reset mode (conventional linear, non-switched) depending on the operating condition. A further advantage of the full control embodiment is that it reduces core size and at the same time reduces the required number of power turns. This embodiment also allows efficient use of any kind of loop material and allows the use of lower cost ferrites at lower frequencies.
An advantage of the present invention is that it improves the operating efficiency of the power converter by minimizing the power loss associated with the control transistor element. Another advantage of the present invention is that it allows the use of lower cost ferrite cores which are lower cost than conventional amorphous cores, run better at high frequencies and can run at higher temperatures. A feature of the present invention is that it is inexpensive to manufacture since magamps have lower parts count and are easier to design than conventional post regulators.